BinaryTreeRightSideViewBFS.java

// BFS with a queue
public List<Integer> rightSideView(TreeNode root) {
    List<Integer> res = new ArrayList<>();
    if (root == null) {
        return res;
    }

    // need to put node in the queue, not value
    Queue<TreeNode> q = new LinkedList<>();
    q.add(root);

    while (!q.isEmpty()) {
        int s = q.size();
        // classic template for bfs, remember the queue size is the current level
        for (int i = 0; i < s; i++) {
            TreeNode t = q.poll();
            // the last element is right side view
            if (i == s - 1) {
                res.add(t.val);
            }
            if (t.left != null) q.offer(t.left);
            if (t.right != null) q.offer(t.right);
        }
    }

    return res;
}